The Pentagon on Friday released a timeline of its response to the Sept. 11 terrorist attack on the U.S. Consulate in Benghazi, Libya, pushing back arguments that the Defense Department did not move fast enough to thwart the assault.

The timeline shows that two to four hours after the attack had begun, the Pentagon ordered forces to “prepare to deploy” to Libya and a staging base in Sigonella, Italy. When those forces actually left their bases was not disclosed.

The timeline shows that the forces arrived in Libya and Sigonella long after the attack was over.

U.S. Ambassador J. Christopher Stevens and three other Americans were killed in the assault.

The release of the Pentagon’s timeline comes a week after the CIA issued its own chronology of the Sept. 11 attack, showing frenzied efforts to evacuate American personnel and fend off terrorists.

• At 4:32 p.m. EDT, a Pentagon operations center notified Defense Secretary Leon E. Panetta and Army Gen. Martin Dempsey, chairman of the Joint Chiefs, after the first attack at the U.S. Consulate had begun at about 3:42 p.m.

• At 5 p.m., Mr. Panetta and Gen. Dempsey met with President Obama at the White House for a regularly scheduled meeting, where Libya was at the top of the agenda, a senior defense official said.

• Between 6 p.m. and 8 p.m., Mr. Panetta held several meetings in the Pentagon with senior officials, Army Gen. Carter Ham, commander of Africa Command, to discuss response options for Benghazi and other contingencies, such as a hostage situation.

During those two hours, Mr. Panetta verbally authorized two Marine platoons in Rota, Spain, to prepare to deploy to Benghazi and Tripoli. He also ordered a special operations force training in Croatia and a U.S.-based special operations force to prepare to deploy to Sigonella.

• At 8:30 p.m., the Pentagon operations center conducted a Benghazi conference call with representatives from affected geographic commands and the four services.

• At 8:39 p.m., Mr. Panetta transmitted written authorization for the forces to deploy to Sigonella and Libya. This did not prevent the forces from preparing before this written authorization, the official said.

The Pentagon did not disclose when the Marine platoons and special operations teams actually left for Libya and Sigonella. A senior defense official said disclosure of that information could hurt national security.

• At about 11:15 p.m., a second attack began at the diplomatic compound and the CIA annex building.

• At 12:05 a.m. EDT on Sept. 12, AFRICOM ordered a C-17 transport aircraft in Germany to prepare to deploy to Libya to help evacuate Americans. It was not specified whether the aircraft was headed to Benghazi or Tripoli.

Two hours later, the first wave of Americans who were under siege departed Benghazi for Tripoli. Approximately three hours after that, the second wave of Americans departed Benghazi for Tripoli.

• At 8:15 a.m., the C-17 departed Germany for Tripoli. It departed Tripoli at 1:17 p.m. with American personnel and the remains of the four slain Americans.

• At 1:57 p.m., the European Command special operations force arrived at Sigonella, after the attacks long were over.

• At 2:56 p.m, one Marine platoon arrived in Tripoli, but the other was still in Rota since the attacks were over by the time it was ready to deploy.

• At 3:28 p.m., the U.S.-based special operations force arrived at Sigonella.